It is particularly well-known from the automotive sector to use pulse-echo methods to sense the driving environment. For example, in the case of parking-assistance systems, ultrasonic transducers are used which emit a scanning pulse and receive it again. The distance between the transducer and the reflecting object is obtained from the echo time.
The mass and the springiness of an acoustic transducer first of all define a self-resonant frequency of the transducer, and secondly, form mechanical energy stores. An oscillatory system results which oscillates post-pulse even after the excitation has ended. The receiving cycle begins after this post-pulse oscillation process has ended. Thus, the post-pulse oscillation defines a dead time during which no receiving is carried out, that is honored when switching over from the transmit mode to the receive mode of the transducer.
For example, German Published Patent Appln. No. 10 2004 020426 mentions the dead time, which results owing to the post-pulse oscillation, as a problem which limits a minimal measuring distance downwards. The citation mentions a distance of 20 cm as minimal measuring distance or measuring limit. In this context, the publication points out that echo signals of shorter distance merge with the post-pulse oscillation and can no longer be detected.
To be sure, it is known to reduce the post-pulse oscillation by additional mechanical damping. However, the overall sensitivity of the transducer is thereby decreased, which is why this measure can only be used to a limited extent.
Nevertheless, there is frequently the desire to provide distance sensors, especially for the automotive sector, which are also able to sense objects at shorter distances.